This invention relates to power supplies in general, and more particularly to a power supply of the type utilizing pulse width modulation techniques to generate DC output voltages at high current levels with increased efficiency.
There are a number of power supplies which include inverters, converters and so on which operate to provide output regulated DC voltages and currents by utilizing switching techniques in lieu of conventional rectifiers and dissapative series pass regulation schemes. These designs employ a number of different circuit configurations but all operate with the common concept that the output voltage is derived from the average value of a rectangular or pulse waveform. By varying the width of this pulse and therefore the average value, one can regulate such supplies.
The advantages in using switching techniques to fabricate power supplies are realized in increased efficiency and smaller size. Efficiency is obtained due to the fact that switching techniques can maintain a constant output as the input line voltage and load changes. The transistors used in switching techniques exhibit low voltage drops when they are on or conducting and conduct very little or no current when they are switched off. With other conventional systems, the variations in line voltage and load changes must be absorbed to maintain a constant output. Thus in a supply using the conventional series pass transistor regulator which acts as a variable resistor in series with the output level, excessive dissipation can occur for varying line and load conditions.
This dissipation is eliminated using switching techniques as the transistors used to generate the constant output are switched on and off at relatively high frequencies. The output waveform produced by the switching transistors is then rectified to provide the desired DC output. The switching duty cycle of operation can be controlled to provide regulation by varying the width of the switching waveforms in accordance with line or load changes. Due to operation at frequencies well above that of the AC line (60Hz), the transformers used in such supplies are smaller and substantially lighter in weight.
In any event, while switching techniques used in power supplies are well known, they impose various burdens on the manufacturer. First, a switching type supply usually has an increased number of components as compared to conventional supplies. They further operate at higher noise levels due to the generation of RFI caused by the sharp transitions of the switching waveforms and the rate of switching in general.
As the frequency of operation becomes higher, the transistors used in developing the switching waveforms do not respond rapidly enough due to emitter to base and collector base capacities and so on, and in essence, may not operate in a switching mode but tend to conduct at the same time. This can short out the transformers which result in the loss of operation. In any event, the higher the frequency of the switching operation, the smaller the magnetic components.
The limit of high frequency with high power operation is inherent in the commercially available transistors. Therefore, a compromise must be made in regard to the switching frequency or the problem of assuring reliable operation has to be solved by the use of compensating circuitry.
The prior art was cognizant of the problem and referenced the compensating circuitry as dead-time circuits and used the same in inverter or power supply configurations.
As indicated, the problem arises where both switching power transistors are conducting or are on at the same time. The problem designated as switch through could cause power loss or switching transistor failure.
U.S. Pat. No. 3,435,324 entitled DRIVEN INVERTER DEAD-TIME CIRCUIT issued on Mar. 25, 1969 shows such a circuit which operates to bias on a transistor, which transistor serves to short out the drive signal long enough to assure all the off-going transistors are able to recover. In any event, the circuit shown is frequency dependent and uses transformer coupling and capacitors in the dead time circuit and hence is not adaptable for wide frequency operation and is relatively expensive to implement.
Other circuits as that shown in U.S. Pat. No. 3,461,405 also use dead time circuitry consisting of saturable core reactors and diodes to permit reliable turn off of the switching transistors. This technique is also expensive and frequency dependent. Other approaches as U.S. Pat. No. 3,506,908 are feedback windings to improve efficiency which techniques again are frequency dependent and unreliable.
Other patents as U.S. Pat. No. 3,629,725 use similar techniques including saturable reactors and diodes to solve the problem. Other configurations as shown in U.S. Pat. No. 3,771,040 show unbalanced circuit configurations which employ multivibrators as duty-cycle generators. The multis are unbalanced to assure that a given one of the transistors is on first, when power is applied. Such unbalancing limits the operation and creates additional problems in implementing the circuitry.
The basic concept of a particular type of switching supply sometimes referred to as a pulse width modulation (PWM) supply is to generate a DC voltage from the input source. A switching transistor is used to switch the DC on and off generating a waveform with excursions between a point of reference potential and the DC level. This waveform is then filtered to provide a DC output component from the repetitive switching waveform.
Besides the above described switch-through problem, many other factors impose certain difficulties.
For example, the input requirements for such supplies generally require the primary circuits to be capable of operating in series or parallel causing a pair or one of the switching transistors to "float" with respect to the input.
Furthermore, one must provide regulation of such power switching supplies. While switching regulators are also known and function to change the switching rate or pulse width, one must condider all of the above problems to provide an efficient and reliable power supply.
It is therefore an object of this invention to provide an improved switching regulator apparatus which apparatus is accomplished by utilizing the dead-time circuitry as a modulating or regulation means.
It is therefore another object of this invention to provide an efficient switching power supply having a dead-time circuit for eliminating switch through, which is relatively easy to fabricate and inexpensive to implement while operating relatively independent of the switching frequency as relying on frequency insensitive components. The switching power supplies include other features which assure reliable operation of such a supply, while maintaining increased efficiency with decreased size.
A further object is to provide a simple means of driving an inverter with a pulse width modulated waveform.
Still another object of the present invention is to provide a power supply having all drive and associated circuits floating to enable series or parallel connections, while providing optimun drive to such circuits.